1. Field of the Invention
This invention relates to an improved process for producing acrylic or methacrylic esters. More particularly, this invention provides an industrially advantageous process for producing acrylic or methacrylic esters in which an acid catalyst used for esterification reaction is efficiently recovered and reused.
2. Background Art
For the industrial production of acrylic or methacrylic esters from acrylic or methacrylic acid and alcohols having 4 or more carbon atoms, the method in which the esterification reaction is carried out by using a strong acid as a catalyst, as described in, for instance, Japanese Laid-Open Patent Publication No. 271247/1986, has been conventionally employed.
The esterification reaction between acrylic or methacrylic acid and an alcohol having 4 or more carbon atoms is an equilibrium reaction. Therefore, the conversion in the reaction cannot exceed a certain degree which is determined by an equilibrium constant. In order to increase the degree of conversion in the reaction, it is necessary to use a large excess of either one of acrylic (or methacrylic) acid and an alcohol (German Patent No. 2548561), or to remove water (reaction water) produced in the reaction from the reaction mixture. The removal of the reaction water is usually effected by distillation. A third component may be added as an azeotropic agent in order to increase the efficiency of the distillation (Japanese Patent Publication No. 41663/1987).
As the strong acids for use as a catalyst, there may be mentioned sulfuric acid, p-toluenesulfonic acid, benzenesulfonic acid, xylenesulfonic acid, naphthenesulfonic acid, methanesulfonic acid, etc. These stong acids must be removed from the reaction liquid after the reaction is completed.
As a post-treatment method for removing the strong acid, there has been proposed a method in which the reaction liquid is treated with an aqueous alkaline solution for neutralization (Japanese Laid-Open Patent Publications Nos. 243046/1986, 34965/1991 and 230240/1992). According to this method, a large amount of alkali is needed for effecting a sufficient neutralization. Moreover, it is very difficult to separate and recover the acid catalyst and unreacted acrylic or methacrylic acid from the aqueous alkaline solution after the treatment and, therefore, the aqueous alkaline solution must be disposed as a waste after the treatment. This method thus has the drawback of the production of a large amount of waste water which contains the harmful organic acid salt at a high concentration.
An object of the present invention is to provide a process for the production of acrylic or methacrylic esters which enables the reuse of an acid catalyst and which overcomes the problem of organic salt-containing waste water encountered in the above prior art.